add NOT_EQUAL to operator mapping (fixes #1603)

NOT_EQUAL was already special-cased as an exclusion operator by
Filter.check(), but the special case was never reached because of the
prior attempt to lookup the operator in the mapping dict, which caused a
KeyError.

python/nav/models/profiles.py

@@ -691,6 +691,7 @@ class Operator(models.Model):
     # cased.
     OPERATOR_MAPPING = {
         EQUALS: '__exact',
+        NOT_EQUAL: '',  # exclusion is special-cased by Filter.check()
         GREATER: '__gt',
         GREATER_EQ: '__gte',
         LESS: '__lt',